Device for monitoring the fatigue life of a structural member and a method of making same

ABSTRACT

A device for monitoring the fatigue life of a structural member composed of a predetermined material, and a method of making same. The device is a substantially flat fuse composed of a material similar to the predetermined material and mountable on a structural member during use. The fuse has cut out portions which define fuse elements which also have reduced thicknesses to effect failure at different times in sequence because of fatigue and prior to the failure of the monitored structural member when the fuse and the monitored structural member are subjected to substantially the same stress history.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to fatigue monitors and more particularly to anovel and highly effective device for detecting fatigue of a monitoredstructure due to flexure and to a method of making the device. Theinvention is applicable particularly to the detection of metal fatiguebut is applicable also to the detection of fatigue in other structuralmaterials.

2. Description of the Prior Art

Maurice A. Brull U.S. Pat. Nos. 4,590,804 and 4,639,997, both assignedto the assignee of the present invention, disclose respectively a devicefor and method of monitoring fatigue life. The patents illustrate whatare called coupons, otherwise known as fatigue fuses, arranged in a rowand each having different notch patterns ranging from the mildest notchpattern to the most aggressive notch pattern. The coupons will failbecause of fatigue in a prescribed sequence. The number of load cyclesto failure as a function of stress amplitude for the different couponscan be plotted, and as each coupon fails, it gives an indication of theremaining life expectancy of the monitored structure.

More particularly, each of the coupons includes a special notch pattern.The coupon axis must be oriented along a suitably chosen direction. Thenotch pattern of each of the coupons produces a stress field whichvaries in intensity from relatively mild to very severe. The severity ofthe local stress field is controlled by the geometry of the notchpattern. Smooth geometries produce a mild stress concentration, whilegeometric discontinuities produce severe stresses. In this manner, whenall of the coupons are subjected to the same stress history, it willresult in the development of different stress concentrations in theregion of the notch tips of each coupon, so that each coupon will have adifferent fatigue life. Moreover, if the stress history of the couponsis the same as that of the monitored structure, the fatigue life of eachcoupon will be a different percentage of the fatigue life of themonitored structure.

In accordance with the disclosures of the patents, the coupons aresecured to the monitored structure by pins, adhesive or welding so that,ideally, all of the coupons experience the same strain history as themonitored structure.

SUMMARY OF THE INVENTION

An object of the present invention is to obtain an improvement over thedevice disclosed in the prior art.

Another object of the present invention is to provide a device formonitoring the fatigue life of a structural member by utilizing afatigue fuse which can be controlled via geometry to more sensitivelydetect failure at a range of percentages of the fatigue life of themonitored structural member.

A still further object of the present invention is to provide a methodof making the above-mentioned devices.

These and other objects of the present invention are achieved inaccordance with the present invention by a device for monitoring thefatigue life of a structural member composed of a predeterminedmaterial, wherein the device is a substantially flat fuse composed ofthe predetermined or similar material and mountable on the structureduring use. The fuse cut out portions define fuse elements. The definedfuse elements have different shapes configured to fail at differenttimes in sequence because of fatigue and prior to the failure of themonitored structural member when the fuse and the monitored structuralmember are subjected to substantially the same stress history.

In accordance with the invention, a cut out portion is formed on atleast one side of each fuse leg, each cut out portion having a differentconfiguration.

The fuse is made more sensitive by a modification that focusses strain(and stress) into the notch area. This can be done in the followingways: thinning the fuse leg along the central portion of the unbondedregion, thickening the fuse leg external to the central portion of theunbonded region, or attaching a stiffer material to the fuse leg in theregion external to the unbonded region.

In a preferred embodiment of the invention, the fuse elements have areduced thickness as compared with the remainder of the fuse in order tocontrol the percentage of the fatigue life that the fuse elements willfail at. The reduced thickness is in the form of a channel which has aconstant depth or a variable depth.

In accordance with another independent aspect of the present invention,a method of making a device for monitoring the fatigue life of astructural member comprises obtaining a thin sheet of material fromwhich the structural member is fabricated and cutting out portions todefine the fuse members.

The device for monitoring the fatigue life according to the presentinvention comprises a substantially flat fuse having means forming atleast one fuse leg, a cut out portion on at least one side of the leg todefine a fuse element, and means effecting a reduced thickness of thefuse element relative to the remainder of the fuse. The device can havea plurality of legs where the cut out portions of the legs havedifferent shapes and the reduced thickness of the fuse element is thesame. This effects failure of the fuse elements at different percentagesof the fatigue life of the structural member to which the device isattached. The fuse elements can have different reduced thicknessesrelative to the remainder of the fuse to effect failure of the fuseelements at different percentages of the fatigue life of the structuralmember and the cut out portions can have the same shape in thisinstance.

The means for effecting a reduced thickness can comprise a channel inone or both faces of the fuse or it can be formed by bonding flatmembers on one face of the fuse on both sides of the fuse elements. Theflat members can be composed of the same material as the fuse or of astiffer material than the fuse.

The reduced thickness area can vary from leg to leg and is in the rangeof 10 to 90% of the remaining thickness of the fuse.

These and other features and advantages of the present invention will beunderstood from the following detailed description from the presentinvention and the accompanying drawing, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a device for monitoring the fatigue life of astructural member in accordance with the present invention;

FIG. 2 is a side view of FIG. 1;

FIG. 3 is a plan view of another embodiment of the device according tothe present invention;

FIG. 4 is a partial sectional view along line IV--IV in FIG. 3;

FIG. 5 is a partial sectional view along line V--V in FIG. 3;

FIG. 6 is a partial sectional view along line VI--VI in FIG. 3;

FIG. 7 is a partial sectional view along line VII--VII in FIG. 3.

FIG. 8 is a plan view of a further embodiment of the device according tothe present invention, and

FIG. 9 is a sectional view of FIG. 8 along line IX--IX.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a device for detecting fatigue of a monitored structure dueto flexure, the monitored structure being made of a predeterminedmaterial. While the monitored structure is not illustrated, it can be apart of a bridge, ship, bulldozer, truck, mobile missile, gun mount,commercial electric power generator (nuclear or conventional), chemicalplant, tower, crane, oil rig, airplane, etc. In an airplane, to takejust one example, the part monitored can be an inboard or outboardstringer splice, a floor system, a wing-fuselage tee, a spar cap splice,etc. In general, the part monitored can be flat, concave or convex.Typically, the predetermined material of which the monitored structureis made will be an aluminum alloy or a steel alloy, although it may beanother metal, a plastic, or any other structural material subject tofailure because of fatigue. The monitored structure and the fuse may bemade, for example, of 7075 aluminum, 6061 aluminum, 2024 aluminum, 1018mild steel, 316 stainless steel, titanium, etc.

The device shown in FIG. 1 comprises a fuse 1 made of the same orsimilar material as the structural member to be monitored and is formedfrom a thin integral blank of sheet material 2. The blank 2 has firstand second elongated slots 3, 4 forming three fuse legs 5, 6, 7. Fuseleg 5 has cut out portion 5a, fuse leg 6 has cut out portion 6a and fuseleg 7 has cut out portion 7a to form fuse elements 8, 9 and 10respectively.

As shown, cut out portion 5a and cut out portion 6a are elongated andcut out portion 7a is a section of a circle.

The three fuse elements 5, 6, 7 which are thus formed, have a differentconfiguration, calculated to fail at different times in sequence becauseof fatigue and prior to the failure of the monitored structural memberwhen the fuse and the monitored structural member are subjected tosubstantially the same stress history.

The percentage of fatigue which defines the time that each fuse memberwill fail is also determined by the reduced thickness or scarf out zone11 which includes a channel 12 formed at one or both faces of the fuse.

The fuse is attached to the structural member by adhesive bonding. Theadhesive acts as an insulator which insulates the fuse from thestructural member. In this way the fuse legs can be remotely monitoredby electrical means to determine if the fuse element correspondingthereto has failed. The method and apparatus for carrying this out isdisclosed in copending U.S. application Ser. No. 07/677,163 filed Mar.29, 1991 now U.S. Pat. No. 5,237,875, and assigned to the same assignee.The disclosure of that application is incorporated by reference herein.

The operation of the fuse is also controlled by the extent to which itis bonded to the structural member. As shown, two bonding lines 13, 14are indicated to demarcate the areas 15 and 16 of the fuse which receiveadhesive and are bonded to the structural member during use.

EXAMPLE 1

The fuse 1 is composed of an aluminum Al 7075 T6 alloy and has a lengthof 2" and a width of 0.8750" and a thickness of 0.04 inches. The radiusof cut out portion 7a is 0.0625". The length of cut out 6a is 0.0625"and the width is 0.002". The length of cut out 5a is 0.0625" and thewidth is 0.025". The radius of the circular portion 5b at the endthereof is 0.0125". The line of symmetry of the cut out portions 5a-7ais midway along the length of the fuse. The slots 3 and 4 are identicaland spaced 0.25" from the edges. They have a length of 1.25" and a widthof 0.0625", with the radius of the circular portions 3a, 4a at the endbeing 0.03125". The channel 12 has a width of 0.075" and a depth of0.012". The bonding lines 13, 14 are 0.85" from the ends of the fuse.

The fuse elements 9, 8 and 10 fail in sequence at different percentagesof the fatigue life of the aluminum alloy.

The fuse 1 as shown in FIG. 1 is formed by obtaining a thin sheet ofmaterial 2 from which the structural member to be monitored isfabricated and cutting out portions 3, 4 and 5a-7a from the sheet ofmaterial by punching on a press or by wire cutting electro-dischargemachinery. The channel is formed by milling on a milling machine.

FIGS. 3-7 show another embodiment of the present invention including afuse 31 made of the same material as the structural member to bemonitored and is formed from a thin integral blank of sheet material 32.The blank has three elongated slots 33, 34, 35 forming four fuse legs36, 37, 38 and 39. Fuse legs 36-39 have cut out portions 36a-39a to formfuse elements 45-48 respectively.

As shown in FIG. 3, cut out portions 36a-39a are elongated terminatingin a semicircular edge.

The four fuse elements 45-48 which are thus formed have the sameconfiguration. The fuse elements are configured to fail at differenttimes in sequence as a result of the variable reduced thickness or scarfout zone 41 formed at the underside thereof. The reduced thickness zone41 includes four separate sections 41a-41d. As shown in FIGS. 4-7, thereduced thickness is achieved by the formation of a channel 42 which hasfour discrete steps therein. Each of the channel portions 42a-42d has adifferent depth, and as a result, each fuse member will fail at adifferent time determined by the depth of the channel.

It should be clear that the fuse will also operate when both thethickness varies and the shape of the cut outs are different. Thereduced thickness can vary from 10 to 90% of the overall thickness ofthe fuse. The channels can be formed on one or both surfaces of thefuse.

The embodiment shown in FIG. 3 is also bonded to a structural memberduring use. For this purpose, bonding lines 43, 44 are shown todemarcate the areas 50, 51 in which adhesive is applied for bonding thefuse to the structural member during use.

EXAMPLE 2

The fuse 31 is composed of an Al 7075 T6 alloy and has a length of 2"and a width of 1.1875" and a thickness of 0.04". Each slot 33-39 isidentical and has a length of 1.25" with the internal end terminating ina circular area having a radius of 0.03125". The width of each slot33-35 is 0.0625" and each leg 36-39 has a width of 0.25".

The cut outs 36a-39a are identical and have a length of 0.0625" and witha radius at the internal end of 0.0125". Each cut out portion 36a-39ahas a width of 0.025".

The bonding lines 43 and 44 are 0.85" from the edges of the fuse.

All of the cut outs and the channel 42 are disposed symmetrically abouta center line of the fuse 31. The channel has a width of 0.075" andchannel section 42a has a depth of 0.018", channel portion 42b has adepth of 0.008", channel portion 42c has a depth of 0.012" and channelportion 42d has a depth of 0.03".

As a result of this configuration, fuse elements 46, 47, 45 and 48 failin sequence at different percentages of the fatigue life of the aluminumalloy.

Fuse 31, as shown in FIG. 3, is formed by obtaining a thin sheet ofmaterial from which the structural member to be monitored is fabricatedand cutting out portions 33-39 and 36a-39a from sheet of material bypunching on a press or by wire cutting electro-discharge machinery. Thevariable channel is formed by milling each section to a different depthon a milling machine.

FIGS. 8 and 9 show another embodiment of the present invention. In thisembodiment, all of the elements shown in FIGS. 8 and 9 which are similarin structure and function to those of FIGS. 1 and 2 are labeled with a"prime".

In the present invention, the reduced thickness zone 11' is not achannel as in the embodiment of FIGS. 1 and 2, but rather is formed bybonding flat members 91 and 93 with adhesive layers 95 and 97,respectively, on the fuse 2'. As a result of the configuration of flatmembers 91 and 93, the space 92 is left which has a reduced thicknessfrom that of the remaining areas of the fuse 2'. The area 92 includesthe fuse members 8'-10' because flat members 91 and 93 are on both sidesof the fuse elements.

The fuse shown in FIGS. 8 and 9 is bonded to a structural member atsponding areas 15' and 16'. The flat members 91 and 93 can be composedof the same material as the fuse 2' or can be made from a stiffermaterial. Thus, if the fuse 2' is made from an aluminum, the flatmembers 91 and 93 can be made from steel or a composite material such asgraphite epoxy.

Many modifications of the preferred embodiments of the inventiondescribed above will readily occur to those skilled in the art uponconsideration of this disclosure. For example, the configuration of thenotches can be changed as well as the dimensions of the fuse member andthe fuse elements thus formed. The number of fuse elements can beincreased or decreased. Accordingly, the invention is not limited exceptas to the following claims.

What is claimed is:
 1. A device for monitoring the fatigue life of astructural member composed of a predetermined material, the devicecomprising: a substantially flat fuse composed of a material of similarcomposition to said predetermined material mountable on the structuralmember, said fuse having means defining at least two fuse legs, each leghaving a cut out portion on at least one side thereof to delimit a fuseelement therein and means forming a reduced thickness of each fuseelement relative to a remainder of the fuse and wherein the fuseelements each have a different reduced thickness, whereby aconfiguration of the cut out portion and reduced thicknesses effect afailure of the fuse elements at different percentages of fatigue life ofthe structural member due to fatigue prior to a failure of thestructural member when the fuse and the structural member are subjectedto a substantially similar stress history.
 2. The device according toclaim 1, wherein the means defining at least two fuse legs comprises atleast one elongated slot.
 3. The device according to claim 2, whereinthe cut out portions of the legs have different shapes.
 4. The deviceaccording to claim 1, wherein the fuse and the structural member arecomposed of the same material.
 5. The device according to claim 1,wherein the cut out portions have the same shape.
 6. The deviceaccording to claim 1, wherein the fuse and the structural member arecomposed of a metal.
 7. The device according to claim 1, wherein themeans forming reduced thicknesses comprises channels in the fuse legs.8. The device according to claim 7, wherein the channel is in onesurface of the fuse.
 9. A method of making a device for monitoring thefatigue life of a structural member composed of a predeterminedmaterial, the method comprising the steps of: providing a substantiallyflat blank fuse composed of a material of similar composition to saidpredetermined material, defining at least two fuse legs by cutting anelongated slot in the fuse, forming at least two fuse elements in thefuse by cutting out portions thereof and reducing the thickness of eachof the fuse elements relative to the remainder of the fuse, wherein thefuse elements each have a different reduced thickness to effect afailure of the fuse elements at different percentages of fatigue life ofthe structural member and prior to a failure of the structural memberwhen the fuse elements and the structural member are subjected to asubstantially similar stress history.
 10. The method according to claim9, wherein the step of reducing comprises reducing the thickness of eachfuse element by a different amount in the range of 10 to 90%.
 11. Themethod according to claim 9, wherein the step of reducing the thicknesscomprises forming a channel in the fuse.
 12. The method according toclaim 11, wherein the channel is formed in one surface of the fuse. 13.The method according to claim 9, wherein the step of reducing comprisesbonding two flat members to one surface of the fuse on opposite sides ofeach fuse element.